Grease traps at commercial establishments, such as fast food restaurants, contain not only grease but also a great deal of water and sediment including sand and dirt. It is common practice to clean out a grease trap using a "vacuum" truck having a tank in which is established a subatmospheric pressure to draw the contents of the trap into the tank through a suction hose. The turbulence caused by the suction mixes the contents of the trap and much sand and dirt is drawn into the tank with the water and grease.
Under certain state solid waste regulations, these contents cannot be taken to a sanitary landfill and dumped because the contents include "free liquids". These are defined as liquids which readily separate from the solid portion of a waste under ambient temperature and pressure. Additionally the contents cannot be readily processed at a typical waste treatment plant because of the grease, which tends to clog various processing equipment thereby increasing maintenance and downtime.
If the grease trap contents can be sufficiently dewatered that the remainder, which is termed "sludge", has at least 15% solids, the solids will hold the liquids so that there are no free liquids. This sludge, with at least 15% solids, is "bladeable", meaning that it can be taken to a sanitary landfill, dumped and spread to dry by means of, for example, a bulldozer.